Communication device, communication method, and communication system

ABSTRACT

A communication device of the disclosure includes a phase synchronizer, a modulator, and a controller. The phase synchronizer generates a second signal on a basis of a first signal received from a communication partner by selectively performing one of a closed loop operation and an open loop operation. The modulator is able to modulate the first signal on a basis of the second signal. The controller controls operations of the phase synchronizer and the modulator.

TECHNICAL FIELD

The disclosure relates to: a communication device used for near fieldcommunication (NFC); a communication method used for such acommunication device; and a communication system including such acommunication device.

BACKGROUND ART

NFC is a technique increasingly used mainly in Asian areas in the fieldsof traffic systems, accounting, and certification, for example. Thistechnique has been approved as an international standard. Thespecification for the NFC, which is also referred to below as the NFCspecification, has compatibility, i.e., so-called upward compatibility,for example, with various other specifications, such as Type A, Type B,FeliCa (registered trademark), and IS015693 specifications. In otherwords, when a reader-writer or a card is compatible with the NFCspecification, it is also compatible with all of those specifications.

In the NFC, as an example, a reader-writer subjects data to an amplitudeshift keying (ASK) modulation and then transmits this data to a card. Inturn, the card subjects data to a load modulation and then transmitsthis data to the reader-writer. PTLs 1 and 2 disclose examples of acommunication device that is able to conduct communication using a loadmodulation, specifically, a passive load modulation.

On the other hand, some electronic circuits use phase locked loop (PLL)circuits, as disclosed in PTL 3, for example.

CITATION LIST Patent Literature

-   PTL 1: Japanese Unexamined Patent Application Publication No.    2013-62605-   PTL 2: Japanese Unexamined Patent Application Publication No.    2011-254156-   PTL 3: Japanese Unexamined Patent Application Publication No.    2012-205137

SUMMARY OF THE INVENTION

In general, communications of enhanced qualities are demanded. Furtherimprovements in communication qualities are expected.

It is desirable to provide a communication device, a communicationmethod, and a communication system that make it possible to enhance acommunication quality.

A communication device according to an embodiment of the disclosureincludes a phase synchronizer, a modulator, and a controller. The phasesynchronizer generates a second signal on a basis of a first signalreceived from a communication partner by selectively performing one of aclosed loop operation and an open loop operation. The modulator is ableto modulate the first signal on a basis of the second signal. Thecontroller controls operations of the phase synchronizer and themodulator.

A communication method according to an embodiment of the disclosureincludes: causing a phase synchronizer to selectively perform one of aclosed loop operation and an open loop operation to generate a secondsignal on a basis of a first signal received from a communicationpartner; and causing a modulator to modulate the first signal on a basisof the second signal.

A communication system according to an embodiment of the disclosureincludes a first communication device and a second communication device.The first communication device transmits a first signal. The secondcommunication device includes a phase synchronizer, a modulator, and acontroller. The phase synchronizer generates a second signal on a basisof the first signal received from the first communication device byselectively performing one of a closed loop operation and an open loopoperation. The modulator is able to modulate the first signal on a basisof the second signal. The controller controls operations of the phasesynchronizer and the modulator.

In the communication device, the communication method, and thecommunication system according to respective embodiments of thedisclosure, the phase synchronizer generates the second signal on thebasis of the first signal received from the communication partner. Thefirst signal is modulated on the basis of this second signal. The abovephase synchronizer selectively performs one of the closed loop operationand the open loop operation.

In the communication device, the communication method, and thecommunication system according to the respective embodiments of thedisclosure, the phase synchronizer selectively performs one of theclosed loop operation and the open loop operation, thereby making itpossible to enhance a communication quality. It is to be noted that theeffects described here are not necessarily limited thereto, and may beany of effects described in the disclosure.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating an example of a configuration ofa communication system according to an embodiment of the disclosure.

FIG. 2 is a block diagram illustrating an example of a configuration ofa phase synchronizer illustrated in FIG. 1.

FIG. 3 is a timing waveform diagram illustrating an example of anoperation of a signal generator according to a first embodiment.

FIG. 4 is an explanatory schematic diagram of a concept of an activeload modulation.

FIG. 5 is a timing waveform diagram illustrating an example of anoperation of a communication system according to a first embodiment.

FIG. 6 is a timing waveform diagram illustrating another example of theoperation of the communication system according to the first embodiment.

FIG. 7 is a characteristic diagram illustrating an example of acharacteristic of the communication system illustrated in FIG. 1.

FIG. 8 is a timing waveform diagram illustrating an example of anoperation of a signal generator according to a modification example ofthe first embodiment.

FIG. 9 is a block diagram illustrating an example of a configuration ofa communication system according to another modification example of thefirst embodiment.

FIG. 10 is a flowchart of an example of an operation of a communicationsystem according to another modification example of the firstembodiment.

FIG. 11 is a flowchart of an example of an operation of a communicationsystem according to another modification example of the firstembodiment.

FIG. 12 is a block diagram illustrating an example of a configuration ofa communication system according to another modification example of thefirst embodiment.

FIG. 13 is a flowchart of an example of an operation of a communicationsystem according to another modification example of the firstembodiment.

FIG. 14 is a waveform diagram illustrating an example of an operation ofa signal generator according to a second embodiment.

FIG. 15 is a timing waveform diagram illustrating an example of anoperation of a communication system according to the second embodiment.

FIG. 16 is a timing waveform diagram illustrating an example of anoperation of a signal generator according to a modification example ofthe second embodiment.

MODES FOR CARRYING OUT THE INVENTION

Some embodiments of the disclosure are described below in detail withreference to the accompanying drawings. The description is given in thefollowing order.

-   1. First Embodiment-   2. Second Embodiment

1. First Embodiment Configuration Example

FIG. 1 illustrates an example of a configuration of a communicationsystem 1 including communication devices according to a firstembodiment. The communication system 1 conducts communication inaccordance with the near field communication. It is to be noted that acommunication method according to an embodiment of the disclosure isalso embodied by the present embodiment, and thus is described together.The communication system 1 includes a communication device 10 and acommunication device 20.

The communication device 10 and the communication device 20 communicatewith each other through a magnetic field. In the communication system 1,specifically, a coil 15 in the communication device 10 is coupled to acoil 21 in the communication device 20 through a magnetic field, therebytransmitting or receiving data by means of electromagnetic induction.The coils 15 and 21 are described later. In this situation, thecommunication device 10 transmits data to the communication device 20 bymeans of an ASK modulation, and the communication device 20 transmitsdata to the communication device 10 by means of a so-called active loadmodulation. The communication device 10 is applicable to reader-writers,for example, and may be compatible with one of various specifications,including Type A, Type B, FeliCa, and IS015693 specifications. Thecommunication device 20 is applicable to cards, for example, and may becompatible with the NFC specification, which has upward compatibilitywith those specifications.

The communication device 10 includes a carrier signal generator 11, adata generator 12, a modulator 13, an amplifier 14, a coil 15, acapacitor 16, and a demodulator 17.

The carrier signal generator 11 generates a carrier signal. The carriersignal may have a frequency of about 13.56 MHz, for example. The datagenerator 12 generates data D1 to be transmitted.

The modulator 13 ASK-modulates the carrier signal by using the data D1,thereby generating a signal SIG11.

The amplifier 14 generates a signal SIG12 on the basis of the signalSIG11, and then outputs this signal SIG12 as an inter-terminal signalbetween a first output terminal and a second output terminal. A firstoutput terminal of the amplifier 14 is coupled to a first end of thecoil 15 and to a first end of the capacitor 16. A second output terminalof the amplifier 14 is coupled to a second end of the coil 15 and to asecond end of the capacitor 16.

The coil 15 generates a magnetic field on the basis of the signal SIG12and is to be coupled to the coil 21, described later, in thecommunication device 20 through the magnetic field. The first end of thecoil 15 is coupled to both the first output terminal of the amplifier 14and the first end of the capacitor 16. The second terminal of the coil15 is coupled to both the second output terminal of the amplifier 14 andthe second end of the capacitor 16. The first end of the capacitor 16 iscoupled to both the first end of the coil 15 and the first outputterminal of the amplifier 14. The second end of the capacitor 16 iscoupled to both the second end of the coil 15 and the second outputterminal of the amplifier 14.

The demodulator 17 performs a demodulation operation on the basis of asignal at the second end of the coil 15, thereby receiving data D2transmitted from the communication device 20. The demodulator 17 may beconfigured using a so-called in-phase/quadrature (I/Q) demodulator, forexample. In the communication system 1, the communication device 20conducts communication by means of a load modulation when transmittingdata to the communication device 10. Specifically, during the period inwhich the communication device 10 transmits the carrier signal to thecommunication device 20, a communication controller 28, described later,in the communication device 20 varies a load viewed from thecommunication device 10 in accordance with the data D2 to betransmitted. This variation in the load emerges, for example, as avariation in an amplitude or phase of a voltage signal at the second endof the coil 15 in the communication device 10. The demodulator 17detects the amplitude or phase of the voltage signal at the second endof the coil 15, thereby receiving the data D2 transmitted from thecommunication device 20. Then, the demodulator 17 supplies the data D2to another block in the communication device 10.

In this example, the demodulator 17 operates on the basis of the signalat the second end of the coil 15; however, the operation of thedemodulator 17 is not limited thereto. As an alternative example, thedemodulator 17 may operate on the basis of a signal at the first end ofthe coil 15 or on the basis of a signal across the coil 15.

The communication device 20 includes the coil 21, a capacitor 22, aphase synchronizer 30, amplifiers 24A and 24B, switches 23A and 23B, adata generator 27, the communication controller 28, a signal generator29, and a demodulator 26.

The coil 21 is to be coupled to the coil 15 in the communication device10 through the magnetic field. The coil 21 generates a signal SIG21corresponding to the signal SIG12 in the communication device 10, bymeans of electromagnetic induction. A first end of the coil 21 iscoupled to both a first end of the capacitor 22 and a first end of theswitch 23A, whereas a second end of the coil 21 is coupled to both asecond end of the capacitor 22 and a first end of the switch 23B. Thefirst end of the capacitor 22 is coupled to both the first end of thecoil 21 and the first end of the switch 23A, whereas the second end ofthe capacitor 22 is coupled to both the second end of the coil 21 and afirst end of the switch 23B.

The phase synchronizer 30 generates a signal SIG30 on the basis of asignal Sin at the second end of the coil 21. The phase synchronizer 30is configured using a phase locked loop (PLL).

FIG. 2 illustrates an example of a configuration of the phasesynchronizer 30. The phase synchronizer 30 includes a phase frequencydetector (PFD) 31, a charge pump 32, a switch 33, a loop filter 34, anda voltage controlled oscillator (VCO) 35.

The PFD 31 compares a phase of the signal Sin and a phase of the signalSIG30 outputted from the VCO 35 with each other. The charge pump 32selectively supplies a current to the loop filter 34 or sinks currentfrom the loop filter 34 on the basis of a comparison result from the PFD31. An output terminal of the charge pump 32 is coupled to an inputterminal of the loop filter 34 via the switch 33. When the switch 33 isbrought into an ON state, the output terminal of the charge pump 32 andthe input terminal of the loop filter 34 are coupled to each other. Theswitch 33 turns on or off on the basis of a loop control signal CT2. Theloop filter 34 specifies a loop response characteristic of the phasesynchronizer 30. The input terminal of the loop filter 34 is coupled tothe output terminal of the charge pump 32 via the switch 33, whereas theoutput terminal of the loop filter 34 is coupled to an input terminal ofthe VCO 35. The VCO 35 oscillates at a frequency corresponding to anoutput voltage of the loop filter 34, thereby generating the signalSIG30.

With the above configuration, the phase synchronizer 30 generates thesignal SIG30 on the basis of the signal Sin. In this situation, thephase synchronizer 30 is able to selectively perform an open loopoperation or a closed loop operation on the basis of a loop controlsignal CTL2.

The amplifier 24A, illustrated in FIG. 1, amplifies the signal SIG30,and then outputs the amplified signal. The amplifier 24B inverts andamplifies the signal SIG30, and then outputs the inverted amplifiedsignal.

The first end of the switch 23A is coupled to both the first end of thecoil 21 and the first end of the capacitor 22, whereas a second end ofthe switch 23A is coupled to an output terminal of the amplifier 24A.The first end of the switch 23B is coupled to both the second end of thecoil 21 and the second end of the capacitor 22, whereas a second end ofthe switch 23B is coupled to an output terminal of the amplifier 24B.Each of the switches 23A and 23B turns on or off on the basis of amodulation control signal CTL1.

With the above configuration, the communication device 20 turns on oroff the switches 23A and 23B on the basis of the modulation controlsignal CTL1, thereby performing the active load modulation.

The data generator 27 generates the data D2 to be transmitted, and thensupplies the data D2 to the communication controller 28. Thecommunication controller 28 generates the modulation control signal CTL1on the basis of the data D2. Then, the communication controller 28supplies the modulation control signal CTL1 to the switches 23A and 23Bas well as to the signal generator 29.

The signal generator 29 generates the loop control signal CTL2 on thebasis of the modulation control signal CTL1, and then supplies this loopcontrol signal CTL2 to the phase synchronizer 30.

FIG. 3 illustrates an example of an operation of the signal generator29. (A) of FIG. 3 illustrates a waveform of the modulation controlsignal CTL1, and (B) of FIG. 3 illustrates a waveform of the loopcontrol signal CTL2. In this example, the switches 23A and 23B are in anOFF state during the period in which the modulation control signal CTL1is at a Low level, and are in an ON state during the period in which themodulation control signal CTL1 is at a High level. Further, the phasesynchronizer 30 performs a closed loop operation during the period inwhich the loop control signal CTL2 is at a Low level, and performs theopen loop operation during the period in which the loop control signalCTL2 is at a High level.

When the modulation control signal CTL1 rises at timing t1, the signalgenerator 29 changes, in response to this rise, the loop control signalCTL2 from the Low level to the High level. When the modulation controlsignal CTL1 falls at timing t2, the signal generator 29 changes, inresponse to this fall, the loop control signal CTL2 from the High levelto the Low level, at timing t3 that is delayed from timing t2 by apredetermined time. In short, the signal generator 29 delays the fallingtiming of the modulation control signal CTL1 by a shift amount Δt,thereby generating the loop control signal CTL2 with its pulse widthbeing greater than that of the modulation control signal CTL1. The phasesynchronizer 30 thereby performs the open loop operation over theperiod, i.e., the period between timing t1 and t3, which corresponds tothe period between timing t1 and t2 in which the switches 23A and 23Bare in the ON state.

In this example, the rising timing of the modulation control signal CTL1and the rising timing of the loop control signal CTL2 are the same aseach other; however, the rising timings of the modulation control signalCTL1 and the loop control signal CTL2 are not limited thereto. As analternative example, the rising timing of the loop control signal CTL2may be delayed from the rising timing of the modulation control signalCTL1 by the delay time of the circuit. As another alternative example,in a case where the signal generator 29 is configured to operate on thebasis of an unillustrated clock signal, the rising timing of the loopcontrol signal CTL2 may be delayed from the rising timing of themodulation control signal CTL1 by time necessary for an operation of thecircuit, such as a setup time.

The demodulator 26, illustrated in FIG. 1, performs a demodulationoperation on the basis of the signal Sin at the second end of the coil21, thereby receiving the data D1 transmitted from the communicationdevice 10. The demodulator 26 is configured using an ASK demodulator.Then, the demodulator 26 supplies the data D1 to another block in thecommunication device 20.

In this example, the phase synchronizer 30 and the demodulator 26operate on the basis of the signal Sin at the second end of the coil 21;however, the operations of the phase synchronizer 30 and the demodulator26 are not limited thereto. As an alternative example, the phasesynchronizer 30 and the demodulator 26 may operate on the basis of thesignal at the first end of the coil 21 or on the basis of the signalacross the coil 21.

Herein, the communication device 20 may correspond to a specific exampleof a “communication device” in an embodiment of the disclosure. Theamplifiers 24A and 24B and the switches 23A and 23B may correspond to aspecific example of a “modulator” in an embodiment of the disclosure.The communication controller 28 and the signal generator 29 maycorrespond to a specific example of a “controller” in an embodiment ofthe disclosure.

Operation and Workings

Next, a description is given of an operation and workings of thecommunication system 1 in the present embodiment.

Summary of Overall Operation

First, the summary of an overall operation of the communication system 1is described with reference to FIG. 1.

The communication device 10 performs the ASK modulation to transmit thedata D1 to the communication device 20. Specifically, in thecommunication device 10, first, the carrier signal generator 11generates the carrier signal, and the data generator 12 generates thedata D1 to be transmitted. The modulator 13 ASK-modulates the carriersignal by using the data D1, thereby generating the signal SIG11. Theamplifier 14 generates the signal SIG12 on the basis of the signalSIG11. The coil 15 generates a magnetic field on the basis of the signalSIG12. In the communication device 20, the coil 21 generates the signalSIG21 on the basis of the magnetic field generated by the coil 15. Thedemodulator 26 performs the demodulation operation on the basis of thesignal Sin at the second end of the coil 21, thereby receiving the dataD1 transmitted from the communication device 10.

The communication device 20 performs the active load modulation totransmit the data D2 to the communication device 10. Specifically,first, the communication controller 28 generates the modulation controlsignal CTL1 on the basis of the data D2 during the period in which thecommunication device 10 transmits the carrier signal to thecommunication device 20. Then, the signal generator 29 generates theloop control signal CTL2 on the basis of the modulation control signalCTL1. The phase synchronizer 30 selectively performs the open loopoperation or the closed loop operation on the basis of the loop controlsignal CTL2, and generates the signal SIG30 on the basis of the signalSin. The amplifier 24A amplifies the signal SIG30 outputted from thephase synchronizer 30, and then outputs the amplified signal. Theamplifier 24B inverts and amplifies the signal SIG30 outputted from thephase synchronizer 30, and then outputs the inverted amplified signal.Each of the switches 23A and 23B turns on or off on the basis of themodulation control signal CTL1. This causes the load viewed from thecommunication device 10 to vary on the basis of the data D2. Thedemodulator 17 in the communication device 10 performs the demodulationoperation on the basis of the signal at the second end of the coil 15,thereby receiving the data D2 transmitted from the communication device20.

Concerning Active Load Modulation

The communication device 20 performs a so-called active load modulationto transmit the data D2 to the communication device 10. Details of thisactive load modulation are described below.

In the active load modulation, the communication controller 28 turns onor off the switches 23A and 23B in accordance with the data D2 duringthe period in which the communication device 10 transmits the carriersignal to the communication device 20. When both the switches 23A and23B are in the ON state, the signal outputted from the amplifier 24A issupplied to the first end of the coil 21, and the signal outputted fromthe amplifier 24B is supplied to the second end of the coil 21. The coil21 thereby generates the magnetic field. In this way, in the active loadmodulation, the coil 21 in the communication device 20 generates themagnetic field during the period in which the communication device 10transmits the carrier signal to the communication device 20, therebyenabling the magnetic field to be greatly varied, thus making itpossible to vary the load viewed from the communication device 10. Onthe basis of the varied load, the demodulator 17 in the communicationdevice 10 performs the demodulation operation, thereby receiving thedata D2 transmitted from the communication device 20. Consequently, forexample, in the communication system 1, it is possible to increase acommunication distance or to downsize antennas, such as the coils 15 and21.

FIG. 3 schematically illustrates an operation in the active loadmodulation. The coil 15 in the communication device 10 generates amagnetic field on the basis of a carrier signal W1. The coil 21 in thecommunication device 20 generates a magnetic field on the basis of asignal W2 that is synchronized with the carrier signal, during theperiod in which the switches 23A and 23B are in the ON state. Then, thedemodulator 17 in the communication device 10 performs the demodulationoperation on the basis of a synthesized signal W3 generated from thecarrier signal W1 and the signal W2. This synthesized signal W3 isexpressed as follows:

A sin(ωt)+B sin(ωt+θ)=√{square root over (A ² +B ²+2AB cos θ)}sin(θ+ϕ)  (1)

where the first term in the left side expresses the carrier signal W1,and the second term in the left side expresses the signal W2. In thisway, the synthesized signal W3 is expressed by a composition theorem ofsinusoidal waves. The amplitude of the synthesized signal W3 variesdepending on an amplitude A of the carrier signal W1 and an amplitude Band phase θ of the signal W2. Accordingly, in some situations, theamplitude of the synthesized signal W3 may become extremely low. In suchsituations, the amplitude of the signal Sin at the second end of thecoil 21 in the communication device 20 may also become low. Thecommunication device 20 controls the phase synchronizer 30 to performthe open loop operation during a period, i.e., an open loop operationperiod P2, as described later, corresponding to a period of the activeload modulation, i.e., a modulation period P1 as described later. Thisreduces the possibility of the phase synchronizer 30 in thecommunication device 20 malfunctioning even in a case where theamplitude of the signal Sin is decreased by the active load modulation.

Operation Example 1

FIG. 5 illustrates an example of an operation of the communicationsystem 1. (A) of FIG. 5 illustrates a waveform of the modulation controlsignal CTL1, (B) of FIG. 5 illustrates a waveform of the loop controlsignal CTL2, and (C) of FIG. 5 illustrates a waveform of the signal Sin.In this example, the communication device 20 performs the active loadmodulation during the modulation period P1. In this situation, the phasesynchronizer 30 performs the open loop operation during the open loopoperation period P2, which corresponds to the modulation period P1.Details of these operations are described below.

At and before timing t11, the communication controller 28 in thecommunication device 20 sets the modulation control signal CTL1 to theLow level, as illustrated in (A) of FIG. 5. This causes both theswitches 23A and 23B to be set to the OFF state. Accordingly, the signalSin corresponding to the carrier signal transmitted from thecommunication device 10 emerges at the second end of the coil 21, asillustrated in (C) of FIG. 5. The signal generator 29 sets the loopcontrol signal CTL2 to the Low level on the basis of the modulationcontrol signal CTL1, as illustrated in (B) of FIG. 5. The phasesynchronizer 30 thereby performs the closed loop operation. That is, thephase synchronizer 30 generates the signal SIG30 on the basis of and insynchronization with the signal Sin.

Next, at timing t11, the communication controller 28 changes themodulation control signal CTL1 from the Low level to the High level, asillustrated in (A) of FIG. 5. This causes the switches 23A and 23B to bechanged to the ON state, and the communication device 20 starts theactive load modulation. Simultaneously, the signal generator 29 changesthe loop control signal CTL2 from the Low level to the High level on thebasis of the modulation control signal CTL1, as illustrated in (B) ofFIG. 5. The phase synchronizer 30 thereby starts the open loopoperation. The phase synchronizer 30 sets the switch 33 to the OFFstate. In this situation, the loop filter 34 maintains the outputvoltage, and therefore the frequency of the signal SIG30 outputted fromthe phase synchronizer 30 is also maintained. In this example, theswitches 23A and 23B are brought into the ON state, thereby causing thecarrier signal transmitted from the communication device 10 and thesignal SIG30 to be synthesized together. As a result, the amplitude ofthe signal Sin decreases with a predetermined time constant, asillustrated in (C) of FIG. 5. Then, the amplitude of the signal Sin iskept small until timing t12.

At timing t12, the communication controller 28 changes the modulationcontrol signal CTL1 from the High level to the Low level, as illustratedin (A) of FIG. 5. This causes the switches 23A and 23B to be changed tothe OFF state. As a result, the amplitude of the signal Sin increaseswith a predetermined time constant to come closer to the amplitude thathas been before timing t11, as illustrated in (C) of FIG. 5.

Next, at timing t13, which is a timing after time corresponding to theshift amount Δt has passed from timing t12, the signal generator 29changes the loop control signal CTL2 from the High level to the Lowlevel, as illustrated in (B) of FIG. 5. The phase synchronizer 30thereby starts the closed loop operation.

The communication system 1 repeats the above operations, thereby causingthe communication device 20 to transmit the data D2 to the communicationdevice 10.

As described above, the communication system 1 causes the phasesynchronizer 30 to perform the open loop operation during the open loopoperation period P2, which corresponds to the modulation period P1. Thismakes it possible to reduce the possibility of the phase synchronizer 30malfunctioning.

In a case where the phase synchronizer 30 is configured to constantlyperform the closed loop operation, for example, decreased amplitude ofthe signal Sin during the modulation period P1 may cause the phasesynchronizer 30 to malfunction. Specifically, in a case where theamplitude of the signal Sin is extremely small, the PFD 31 in the phasesynchronizer 30 fails to detect a transition of the signal Sin, and thusfails to perform the operation of comparing the phases. This causes theoutput voltage of the loop filter 34 to gradually deviate from a desiredvoltage. As a result, the frequency of the signal SIG30 deviates from adesired frequency. This may hinder the phase synchronizer 30 fromreturning the frequency of the signal SIG30 to the desired frequency,even when the amplitude of the signal Sin increases after the modulationperiod P1 has ended. Due to the frequency deviation, a frequency of aradio signal in use may deviate from a frequency band that is availablein the near field communication. Eventually, it may become difficult tosatisfy the specification for the near field communication.

On the other hand, the communication system 1 causes the phasesynchronizer 30 to perform the open loop operation during the open loopoperation period P2, which corresponds to the modulation period P1.Accordingly, even in a case where the amplitude of the signal Sin isdecreased by the active load modulation, as illustrated in FIG. 5, theswitch 33 in the phase synchronizer 30 is kept in the OFF state. Thus,the output voltage of the loop filter 34 is maintained. As a result, thefrequency of the signal SIG30 is also maintained. Then, when startingthe closed loop operation, the phase synchronizer 30 is able tosynchronize the signals SIG30 with the signal Sin more securely. In thisway, in the communication device 20, it becomes possible to reduce thepossibility of the phase synchronizer 30 malfunctioning. Consequently,it is possible to enhance a quality of communication in thecommunication system 1.

In the communication system 1, the timing at which the open loopoperation period P2 ends is set to be delayed from the timing at whichthe modulation period P1 ends. This also makes it possible to reduce thepossibility of the phase synchronizer 30 malfunctioning.

That is, in a case where, for example, the phase synchronizer 30 isconfigured to selectively perform the open loop operation or the closedloop operation on the basis of the modulation control signal CTL1,without providing the signal generator 29, the phase synchronizer 30starts the closed loop operation at timing t12. In this case, the phasesynchronizer 30 may malfunction, because the amplitude of the signal Sinis still small right after timing t12.

In contrast, in the communication system 1, the timing at which the openloop operation period P2 ends is set to be delayed from the timing atwhich the modulation period P1 ends. In other words, the signalgenerator 29 delays the falling timing of the modulation control signalCTL1 by the shift amount Δt, thereby generating the loop control signalCTL2. In the communication system 1, as a result, the phase synchronizer30 starts the closed loop operation at timing t13 at which the amplitudeof the signal Sin has increased. This makes it possible to reduce thepossibility of the phase synchronizer 30 malfunctioning. Consequently,it becomes possible to enhance a quality of communication in thecommunication system 1.

Operation Example 2

FIG. 6 illustrates another example of an operation of the communicationsystem 1. (A) of FIG. 6 illustrates a waveform of the modulation controlsignal CTL1, (B) of FIG. 6 illustrates a waveform of the loop controlsignal CTL2, and (C) of FIG. 6 illustrates a waveform of the signal Sin.

As is the case with Operation Example 1 illustrated in FIG. 5, thecommunication controller 28 in the communication device 20 sets themodulation control signal CTL1 to the Low level before timing t21, asillustrated in (A) of FIG. 6. The signal generator 29 sets the loopcontrol signal CTL2 to the Low level, as illustrated in (B) of FIG. 6.The signal Sin corresponding to the carrier signal transmitted from thecommunication device 10 emerges at the second end of the coil 21, asillustrated in (C) of FIG. 6.

At timing t21, the communication controller 28 changes the modulationcontrol signal CTL1 from the Low level to the High level, as illustratedin (A) of FIG. 6. This causes the switches 23A and 23B to be changed tothe ON state, and the communication device 20 starts the active loadmodulation. Simultaneously, the signal generator 29 changes the loopcontrol signal CTL2 from the Low level to the High level on the basis ofthe modulation control signal CTL1, as illustrated in (B) of FIG. 6. Thephase synchronizer 30 thereby starts the open loop operation. In thisexample, the switches 23A and 23B are brought into the ON state, therebycausing the carrier signal transmitted from the communication device 10and the signal SIG30 to be synthesized together. As a result, theamplitude of the signal Sin increases with a predetermined timeconstant, as illustrated in (C) of FIG. 6. Then, the amplitude of thesignal Sin is kept large until timing t22.

Next, at timing t22, the communication controller 28 changes themodulation control signal CTL1 from the High level to the Low level, asillustrated in (A) of FIG. 6. This causes the switches 23A and 23B to bechanged to the OFF state. As a result, the amplitude of the signal Sindecreases with a predetermined time constant, as illustrated in (C) ofFIG. 6. In this situation, the amplitude of the signal Sin transientlybecomes extremely small, as indicated by a waveform W1, and thengradually increases to come closer to the amplitude that has been beforetiming t21.

Next, at timing t23, which is a timing after time corresponding to theshift amount Δt has passed from timing t22, the signal generator 29changes the loop control signal CTL2 from the High level to the Lowlevel, as illustrated in (B) of FIG. 6. The phase synchronizer 30thereby starts the closed loop operation.

The communication system 1 repeats the above operations, thereby causingthe communication device 20 to transmit the data D2 to the communicationdevice 10.

In this example, the amplitude of the signal Sin increases during themodulation period P1, but decreases after the modulation period P1 hasended. In this situation, the amplitude of the signal Sin transientlybecomes extremely small after the modulation period P1 has ended, asindicated by the waveform W1. In the communication system 1, the timingat which the open loop operation period P2 ends is set to timing t23,which is a timing after the waveform W1 has emerged. This prevents thephase synchronizer 30 from performing the closed loop operation withinthe period in which the amplitude of the signal Sin is extremely small.This makes it possible to reduce the possibility of the phasesynchronizer 30 malfunctioning. Consequently, it is possible to enhancea quality of communication in the communication system 1.

Concerning Communication Distance

In the communication system 1, as described above, the signal generator29 delays the falling timing of the modulation control signal CTL1 bythe shift amount Δt, thereby generating the loop control signal CTL2. Byincreasing the shift amount Δt in the communication system 1, it ispossible to increase its communication distance, as described below.

FIG. 7 illustrates a communication characteristic of the communicationsystem 1. The horizontal axis represents the shift amount Δt, and thevertical axis represents the communication distance. In a case where thecommunication distance increases, an amplitude A of the carrier signalW1 illustrated in FIG. 4 decreases. Accordingly, the amplitude of thesynthesized signal W3 varies depending on the communication distance. Ina case where the communication distance increases in this manner,increasing the shift amount Δt makes it possible to reduce thepossibility of the phase synchronizer 30 malfunctioning. In this manner,in the communication system 1, by increasing the shift amount Δt, it ispossible to increase the communication distance.

Effect

In the present embodiment, as described above, the phase synchronizerperforms the open loop operation during the open loop operation periodcorresponding to the load modulation. This makes it possible to reducethe possibility of the phase synchronizer malfunctioning. Consequently,it is possible to enhance a communication quality.

In the present embodiment, by setting the timing at which the open loopoperation period ends to be delayed from the timing at which amodulation period ends, it is possible to reduce the possibility of thephase synchronizer malfunctioning, thus enhancing a communicationquality.

The foregoing embodiment successfully increases a communication distanceby reducing the possibility of the phase synchronizer malfunctioning.

Modification Example 1-1

In the foregoing embodiment, the signal generator 29 delays the fallingtiming of the modulation control signal CTL1 by the shift amount Δt,thereby generating the loop control signal CTL2. This shift amount Δtmay be configured to be variable, as illustrated in FIG. 8. A signalgenerator 29A according to the present modification example is able toset the falling timing of the loop control signal CTL2 to one of timingst31 to t38. Although the falling timing of the loop control signal CTL2may be set to one of the eight timings in this example, the number oftimings settable is not limited thereto. Alternatively, the fallingtiming of the loop control signal CTL2 may be set to one of seven orless timings, or to one of nine or more timings.

Modification Example 1-2

Although the shift amount Δt is fixed in the foregoing embodiment, theshift amount Δt is not limited thereto. As an alternative example, theshift amount Δt may be changed depending on a communication state.Details of the present modification example are described below.

FIG. 9 illustrates an example of a configuration of a communicationsystem 1B according to the present modification example. Thecommunication system 1B includes a communication device 20B. Thecommunication device 20B includes a demodulator 26B and a signalgenerator 29B.

The demodulator 26B performs a demodulation operation on the basis ofthe signal Sin at the second end of the coil 21. In addition, thedemodulator 26B identifies a specification with which the communicationdevice 10 is compliant, i.e., a communication specification SP. Forexample, this identification method may use a modulation degree of asignal transmitted from the communication device 10. That is, asdescribed above, the communication device 10 may be compliant with oneof various specifications, including Type A, Type B, FeliCa, andIS015693 specifications. The modulator 13 in the communication device 10uses a modulation degree corresponding to its compliant specificationwhen performing the ASK modulation. Thus, the demodulator 26B is able toidentify the specification with which the communication device 10 iscompliant, i,e., the communication specification SP on the basis of themodulation degree of the signal transmitted from the communicationdevice 10. However, the identification method is not limited thereto.Alternatively, the demodulator 26B may identify the communicationspecification SP on the basis of another feature of the signal, such ascoding, a bit rate, or a subcarrier. For example, in a case where thedata D1 transmitted by the communication device 10 contains informationin accordance with the communication specification SP, the demodulator26B may identify the communication specification SP on the basis of thisinformation.

Similarly to the above signal generator 29A according to ModificationExample 1-1, the signal generator 29B is configured to set the shiftamount Δt to a variable value. This signal generator 29B sets the shiftamount Δt on the basis of the communication specification SP.

FIG. 14 illustrates an example of an operation of the communicationsystem 2B. In the communication system 2B, first, the communicationdevice 10 transmits the data D1 to the communication device 20B. Then,the communication device 20B responds to this by transmitting the dataD2 to the communication device 10. Details of this operation aredescribed below.

At Step S1, the communication device 10 performs the ASK modulation totransmit the data D1 to the communication device 20B.

At Step S2, the demodulator 26B in the communication device 20B checkswhether the data D1 is received. In a case where the demodulator 26Breceives the data D1, the data generator 27 in the communication device20B generates the data D2 in order to respond to the communicationdevice 10. Then, the operation proceeds to step S3. In a case where thedemodulator 26B fails to receive the data D1, the operation returns tostep S1, and steps S1 and S2 are repeated until the data D1 is received.

At step S3, a demodulator 27B in the communication device 20B identifiesthe communication specification SP. At step S4, the signal generator 29Bin the communication device 20B sets the shift amount Δt on the basis ofthe communication specification SP identified at step S3. Next, at stepS5, the communication device 20B performs the active load modulation totransmit the data D2 to the communication device 10.

Through the above steps, this flow is completed.

In this manner, the communication system 2B sets the shift amount Δt onthe basis of the communication specification SP. This makes it possibleto enhance degree of flexibility when setting the shift amount Δt.Specifically, when the communication specification SP is a specificationintended for long-distance communication, for example, the shift amountΔt may be set to a large value in order to increase a communicationdistance. Consequently, it is possible to enhance a quality ofcommunication in the communication system 2B.

Although the shift amount Δt is set on the basis of the communicationspecification SP as described above, a method of setting the shiftamount Δt is not limited thereto. As an alternative example, the shiftamount Δt may be set on the basis of a bit rate BR. Next, acommunication system 1C according to the present modification example isdescribed.

As illustrated in FIG. 9, the communication system 1C includes acommunication device 20C. The communication device 20C includes ademodulator 26C and a signal generator 29C. The demodulator 26C performsa demodulation operation on the basis of the signal Sin at the secondend of the coil 21. In addition, the demodulator 26B identifies a bitrate BR, which is a bit rate at which the communication device 10 andthe communication device 20C communicate with each other. In thisidentification method, for example, the demodulator 26C detects afrequency of a signal transmitted from the communication device 10, thusacquiring the bit rate BR. For example, in a case where data D1transmitted from the communication device 10 contains information inaccordance with the bit rate BR, the demodulator 26C may identify thebit rate BR on the basis of this information. The signal generator 29Csets the shift amount Δt on the basis of the bit rate BR.

FIG. 11 illustrates an example of an operation of the communicationsystem 1C. At step S1, the communication device 10 performs the ASKmodulation to transmit data D1 to the communication device 20C, as isthe case with the communication system 1B illustrated in FIG. 10. Atstep S2, the demodulator 26C in the communication device 20C checkswhether the data D1 have been received. Next, at step S13, thedemodulator 26C in the communication device 20C identifies the bit rateBR. At step S14, the signal generator 29C in the communication device20C sets the shift amount Δt on the basis of the bit rate BR identifiedat step S13. Next, at step S5, the communication device 20C performs theactive load modulation to transmit data D2 to the communication device10, as is the case with the communication system 1B illustrated in FIG.10.

The communication system 2C sets the shift amount Δt on the basis of thebit rate BR. This makes it possible to enhance degree of flexibilitywhen setting the shift amount Δt. Specifically, when the bit rate BRbecomes higher, for example, the shift amount Δt may preferably be setto a smaller value, because each transmission symbol is handled within ashort period of time. Consequently, it is possible to enhance a qualityof communication in the communication system 2C.

Although the shift amount Δt is set on the basis of the bit rate BR asdescribed above, a method of setting the shift amount Δt is not limitedthereto. As an alternative example, the shift amount Δt may be set onthe basis of the voltage amplitude of the signal Sin. Next, acommunication system 1D according to the present modification example isdescribed.

FIG. 12 illustrates an example of a configuration of the communicationsystem 1D. The communication system 1D includes a communication device20D. The communication device 20D includes an amplitude detector 25D anda signal generator 29D. The amplitude detector 25D detects an amplitudevalue Vsw of the signal Sin on the basis of the signal Sin. The signalgenerator 29D sets a shift amount At on the basis of the amplitude valueVsw.

FIG. 13 illustrates an example of an operation of the communicationsystem 1D. At step S1, the communication device 10 performs the ASKmodulation to transmit data D1 to the communication device 20D, as isthe case with the communication system 1B illustrated in FIG. 10. Atstep S2, the demodulator 26 in the communication device 20D checkswhether the data D1 have been received. Next, at step S23, the amplitudedetector 25D in the communication device 20D detects the amplitude valueVsw of the signal Sin.

Next, at step S24, the signal generator 29D in the communication device20D compares the amplitude value Vsw detected at step S23 with a presetthreshold Vth. In a case where the amplitude value Vsw is equal to ormore than the threshold Vth (Vsw≥Vth), the signal generator 29D sets theshift amount Δt to a shift amount Δt1 at step S25. In a case where theamplitude value Vsw is less than the threshold Vth (Vsw<Vth), the signalgenerator 29D sets the shift amount Δt to a shift amount Δt2 at stepS26.

Next, at step S5, the communication device 20D performs the active loadmodulation to transmit data D2 to the communication device 10, as is thecase with the communication system 1B illustrated in FIG. 10.

In this manner, the communication system 2D sets the shift amount Δt onthe basis of the amplitude value Vsw of the signal Sin. This makes itpossible to enhance degree of flexibility when setting the shift amountΔt. Specifically, when the amplitude value Vsw of the signal Sin becomesmaller, for example, the shift amount Δt may preferably be set to alarger value, because a communication distance between the communicationdevice 10 and the communication device 20D is long. Consequently, it ispossible to enhance a quality of communication in the communicationsystem 2D.

Although the communication system 2D compares the amplitude value Vsw ofthe signal Sin with a single threshold Vth, the comparison method is notlimited thereto. The comparison may also be made using a plurality ofthresholds. This makes it is possible to finely set the shift amount Δt.When the amplitude value Vsw of the signal Sin is sufficiently large,the shift amount Δt may be set to zero.

Alternatively, the shift amount Δt may be set on the basis of two ormore of the communication specification SP, the bit rate BR, and theamplitude value Vsw of the signal Sin.

2. Second Embodiment

Next, a communication system 2 according to a second embodiment isdescribed. A signal generator in the present embodiment operates in adifferent manner from the foregoing first embodiment. It is to be notedthat same components as those of the communication system 1 according tothe foregoing first embodiment are denoted by same reference numerals,and description thereof is omitted as appropriate.

As illustrated in FIG. 1, the communication system 2 includes acommunication device 40. The communication device 40 includes a signalgenerator 49. The signal generator 49 generates the loop control signalCTL2 on the basis of the modulation control signal CTL1, and thensupplies the loop control signal CTL2 to the phase synchronizer 30.

FIG. 14 illustrates an example of an operation of the signal generator49. (A) of FIG. 14 illustrates a waveform of the modulation controlsignal CTL1, and (B) of FIG. 14 illustrates a waveform of the loopcontrol signal CTL2. When the modulation control signal CTL1 rises attiming t41, the signal generator 49 changes, in response to this rise,the loop control signal CTL2 from a Low level to a High level at timingt42 at which predetermined time has passed from the rising timing t41.When the modulation control signal CTL1 falls at timing t43, the signalgenerator 49 changes, in response to this fall, the loop control signalCTL2 from the High level to the Low level at timing t44 at which thepredetermined time has passed from the falling timing t43. In otherwords, the signal generator 49 delays the rising timing and the fallingtiming of the modulation control signal CTL1 by a shift amount Δt,thereby generating the loop control signal CTL2.

FIG. 15 illustrates an example of an operation of the communicationsystem 2. (A) of FIG. 15 illustrates the waveform of the modulationcontrol signal CTL1, (B) of FIG. 15 illustrates the waveform of the loopcontrol signal CTL2, and (C) of FIG. 15 illustrates a waveform of asignal Sin. FIG. 15 corresponds to Operation Example 1, illustrated inFIG. 5, according to the first embodiment.

At timing t51, the communication controller 28 changes the modulationcontrol signal CTL1 from the Low level to the High level, as illustratedin (A) of FIG. 15. This causes the switches 23A and 23B to be changed tothe ON state, and the communication device 40 starts the active loadmodulation. As a result, the carrier signal transmitted from thecommunication device 10 and the signal SIG30 are synthesized together,and the amplitude of the signal Sin decreases with a predetermined timeconstant, as illustrated in (C) of FIG. 15. The amplitude of the signalSin is kept small until timing t53.

Next, at timing t52 at which time corresponding to the shift amount Δthas passed from timing t51, the signal generator 49 changes the loopcontrol signal CTL2 from the Low level to the High level, as illustratedin (B) of FIG. 15. This causes the phase synchronizer 30 to start theopen loop operation. The switch 33 in the phase synchronizer 30 is setto the OFF state. In this situation, the loop filter 34 maintains anoutput voltage, and thus a frequency of the signal SIG30 outputted fromthe phase synchronizer 30 is maintained.

Next, at timing t53, the communication controller 28 changes themodulation control signal CTL1 from the High level to the Low level, asillustrated in (A) of FIG. 15. This causes the switches 23A and 23B tobe changed to the OFF state. As a result, the amplitude of the signalSin increases with a predetermined time constant to come closer to theamplitude that has been before timing t51, as illustrated in (C) of FIG.15.

Next, at timing t54 at which time corresponding to the shift amount Δthas passed from timing t53, the signal generator 49 changes the loopcontrol signal CTL2 from the High level to the Low level, as illustratedin (B) of FIG. 15. The phase synchronizer 30 thereby starts the closedloop operation.

The communication system 2 repeats the above operations, thereby causingthe communication device 40 to transmit the data D2 to the communicationdevice 10.

In the communication system 2, as described above, the timing at whichthe open loop operation period P2 starts is set to be delayed from thetiming at which the modulation period P1 starts. This enables the phasesynchronizer 30 to perform the closed loop operation during the periodbetween timing t51 and t52. That is, the amplitude of the signal Singradually decreases in this period, but the amplitude is still largeenough for the phase synchronizer 30 to be able to perform the closedloop operation. Therefore, the communication system 2 causes the phasesynchronizer 30 to operate during this period. This makes it possible toextend a period of time during which the phase synchronizer 30 performsthe closed loop operation. This enables the signal SIG30 to besynchronized with the signal Sin more securely. Consequently, it ispossible to enhance a quality of communication in the communicationsystem 2.

In the present embodiment, as described above, the timing at which theopen loop operation period starts is set to be delayed from the timingat which the modulation period starts. Thus, it is possible to extendthe period of time during which the phase synchronizer performs theclosed loop operation, thus making it possible to enhance acommunication quality. Other effects are similar to those of theforegoing first embodiment.

Modification Example 2-1

In the foregoing embodiment, the signal generator 49 delays the risingtiming and the falling timing of the modulation control signal CTL1 bythe shift amount Δt, thereby generating the loop control signal CTL2.This shift amount Δt may be configured to be variable, as illustrated inFIG. 16. A signal generator 49A according to the present modificationexample is able to set the rising timing of the loop control signal CTL2to one of timings t61 to 64, and to set the falling timing of the loopcontrol signal CTL2 to one of timings t71 to t74. Although each of therising timing and the falling timing of the loop control signal CTL2 isable to be set to one of the four timings in this example, the number oftimings settable is not limited thereto. Alternatively, each of therising timing and the falling timing of the loop control signal CTL2 maybe set to one of three or less timings or to one of five or moretimings.

The technology has been described using some embodiments andmodification examples. However, the technology is not limited to theseembodiments and modification examples, and may be modified in a varietyof ways.

In each second embodiment, a signal generator 48 delays the risingtiming and the falling timing of the modulation control signal CTL1 bythe shift amount Δt, thereby generating the loop control signal CTL2.However, the method of generating the loop control signal CTL2 is notlimited thereto. As an alternative example, the signal generator 49 maydelay the rising timing of the modulation control signal CTL1 by a shiftamount Δta, and may delay the falling timing of the modulation controlsignal CTL1 by a shift amount Δtb, thereby generating the loop controlsignal CTL2. In this manner, the present modification example makes itpossible to enhance the flexibility of an operation by differentlysetting the shift amount Δta during the rising and the shift amount Δtbduring the falling.

It is to be noted that the effects described herein are merely examplesand are not limitative, and may further include other effects.

It is to be noted that the technology may have the followingconfigurations.

(1)

A communication device including:

a phase synchronizer that generates a second signal on a basis of afirst signal received from a communication partner by selectivelyperforming one of a closed loop operation and an open loop operation;

a modulator that is able to modulate the first signal on a basis of thesecond signal; and

a controller that controls operations of the phase synchronizer and themodulator.

(2)

The communication device according to (1), in which

the controller causes the modulator to modulate the first signal duringa modulation period, and

the controller causes the phase synchronizer to perform the open loopoperation during an open loop operation period, the open loop operationperiod corresponding to the modulation period.

(3)

The communication device according to (2), in which a timing at whichthe modulation period ends is within the open loop operation period.

(4)

The communication device according to (2) or (3), in which

the controller generates a modulation control signal, the modulationcontrol signal being at a first logic level during the modulation periodand at a second logic level during a period other than the modulationperiod,

the controller generates a loop control signal on a basis of themodulation control signal, the loop control signal being at a thirdlogic level during the open loop operation period and at a fourth logiclevel during a period other than the open loop operation period,

the modulator modulates the first signal on the basis of the modulationcontrol signal, and

the phase synchronizer selectively performs one of the closed loopoperation and the open loop operation on a basis of the loop controlsignal.

(5)

The communication device according to (4), in which the controllergenerates the loop control signal by delaying a first transition timingfrom the first logic level of the modulation control signal to thesecond logic level of the modulation control signal.

(6)

The communication device according to (5), in which the controllerdetermines an amount by which the first transition timing is delayed, ona basis of a method of communicating with the communication partner.

(7)

The communication device according to (5) or (6), in which thecontroller determines an amount by which the first transition timing isdelayed, on a basis of a communication rate of communication with thecommunication partner.

(8)

The communication device according to any one of (5) to (7), in whichthe controller determines an amount by which the first transition timingis delayed, on a basis of a signal amplitude of the first signal.

(9)

The communication device according to any one of (5) to (8), in whichthe controller further generates the loop control signal by delaying asecond transition timing from the second logic level of the modulationcontrol signal to the first logic level of the modulation controlsignal.

(10)

The communication device according to any one of (1) to (9), in which

the modulator has a switch,

the phase synchronizer receives the first signal from the communicationpartner via a coil, and

the switch is brought into an ON state to supply the second signal tothe coil.

(11)

A communication method including:

causing a phase synchronizer to selectively perform one of a closed loopoperation and an open loop operation to generate a second signal on abasis of a first signal received from a communication partner, and

causing a modulator to modulate the first signal on a basis of thesecond signal.

(12)

A communication system including:

a first communication device that transmits a first signal; and

a second communication device,

the second communication device including

a phase synchronizer that generates a second signal on a basis of thefirst signal received from the first communication device by selectivelyperforming one of a closed loop operation and an open loop operation,

a modulator that is able to modulate the first signal on a basis of thesecond signal, and

a controller that controls operations of the phase synchronizer and themodulator.

This application is based upon and claims the benefit of priority of theJapanese Patent Application No. 2015-138445 filed with the Japan PatentOffice on Jul. 10, 2015, the entire contents of which are incorporatedherein by reference.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

1. A communication device comprising: a phase synchronizer thatgenerates a second signal on a basis of a first signal received from acommunication partner by selectively performing one of a closed loopoperation and an open loop operation; a modulator that is able tomodulate the first signal on a basis of the second signal; and acontroller that controls operations of the phase synchronizer and themodulator.
 2. The communication device according to claim 1, wherein thecontroller causes the modulator to modulate the first signal during amodulation period, and the controller causes the phase synchronizer toperform the open loop operation during an open loop operation period,the open loop operation period corresponding to the modulation period.3. The communication device according to claim 2, wherein a timing atwhich the modulation period ends is within the open loop operationperiod.
 4. The communication device according to claim 2, wherein thecontroller generates a modulation control signal, the modulation controlsignal being at a first logic level during the modulation period and ata second logic level during a period other than the modulation period,the controller generates a loop control signal on a basis of themodulation control signal, the loop control signal being at a thirdlogic level during the open loop operation period and at a fourth logiclevel during a period other than the open loop operation period, themodulator modulates the first signal on the basis of the modulationcontrol signal, and the phase synchronizer selectively performs one ofthe closed loop operation and the open loop operation on a basis of theloop control signal.
 5. The communication device according to claim 4,wherein the controller generates the loop control signal by delaying afirst transition timing from the first logic level of the modulationcontrol signal to the second logic level of the modulation controlsignal.
 6. The communication device according to claim 5, wherein thecontroller determines an amount by which the first transition timing isdelayed, on a basis of a method of communicating with the communicationpartner.
 7. The communication device according to claim 5, wherein thecontroller determines an amount by which the first transition timing isdelayed, on a basis of a communication rate of communication with thecommunication partner.
 8. The communication device according to claim 5,wherein the controller determines an amount by which the firsttransition timing is delayed, on a basis of a signal amplitude of thefirst signal.
 9. The communication device according to claim 5, whereinthe controller further generates the loop control signal by delaying asecond transition timing from the second logic level of the modulationcontrol signal to the first logic level of the modulation controlsignal.
 10. The communication device according to claim 1, wherein themodulator has a switch, the phase synchronizer receives the first signalfrom the communication partner via a coil, and the switch is broughtinto an ON state to supply the second signal to the coil.
 11. Acommunication method comprising: causing a phase synchronizer toselectively perform one of a closed loop operation and an open loopoperation to generate a second signal on a basis of a first signalreceived from a communication partner, and causing a modulator tomodulate the first signal on a basis of the second signal.
 12. Acommunication system comprising: a first communication device thattransmits a first signal; and a second communication device, the secondcommunication device including a phase synchronizer that generates asecond signal on a basis of the first signal received from the firstcommunication device by selectively performing one of a closed loopoperation and an open loop operation, a modulator that is able tomodulate the first signal on a basis of the second signal, and acontroller that controls operations of the phase synchronizer and themodulator.